1. Field of Invention
This invention relates to wind turbines and more specifically, to an improved vertical-axis wind turbine. (VAWT).
2. Description of Related Art
A wind turbine or wind generator is a rotating machine that converts the kinetic energy in wind into electric energy via gears and generators, or other forms of energy usable by men. Wind energy offers many advantages, which explains why it is the fastest-growing alternative energy source in the world. Because wind energy is fueled by the wind, which in turn is fueled by the sun, it is a clean and inexhaustible fuel source. Wind energy does not pollute the atmosphere like power plants that rely on combustion of fossil fuels such as coal or gas. Wind turbines also do not produce atmospheric emissions that cause acid rain or greenhouse gasses. Wind energy is a domestic source of energy and decreases a nation's dependence on foreign supplied energy such as oil, and creates local well paying jobs. Wind supply is readily abundant in most areas. Wind energy relies on the renewable power of the wind, which cannot be exhausted. Wind energy is also one of the lowest-priced renewable energy technologies available today, costing between 4 and 8 cents per kilowatt-hour, depending upon the wind resource and project financing of the particular project.
Generally, wind turbines can be separated into two types based by the axis in which the turbine axis rotates. Turbines that rotate around a horizontal axis are referred to as horizontal-axis wind turbines (“HAWT”). They are the most common type of wind turbine in use today and are typically seen in open spaces along highways in areas such as Palm Springs and Tehachapi, Calif. Turbines that rotate around a vertical axis are referred to as vertical-axis wind turbines (“VAWT”) and are less frequently used because they are less efficient.
A HAWT typically has a main rotor shaft and an electrical generator mounted at the top of a support tower, and must be pointed into the wind. Small turbines are pointed by a simple wind vane, while large turbines generally use a wind sensor coupled with a servo motor that rotates the turbine and all its components to face the wind. Most turbines have a gearbox, which turns the slow rotation of the blades into a quicker rotation that is more suitable to drive an electrical generator. Since the tower produces turbulence behind it, the turbine is usually pointed upwind of the tower. Turbine blades are made stiff to prevent the blades from being pushed into the tower by high winds. Additionally, the blades are placed a considerable distance in front of the tower and are sometimes tilted up a small amount to prevent them from hitting the towers in high winds. In very strong winds the blades have to be stopped all together to prevent the blades from hitting the tower. Downwind machines have been built, despite the problem of turbulence (mast wake), because they do not need an additional mechanism for keeping them in line with the wind, and because in high winds the blades can be allowed to bend which reduces their swept area and thus their wind resistance. Since cyclic turbulence may lead to fatigue failures most HAWTs are upwind machines.
HAWTs suffer from a number of additional disadvantages. The tall towers and blades up to 90 meters or longer and are difficult to transport. Transportation can reach 20% of equipment costs. In addition, tall HAWTs are difficult to install, needing very tall and expensive cranes and skilled operators. Massive tower and foundations construction is required to support the heavy blades, gearbox, and generator. Their height makes them obtrusively visible across large areas, disrupting the appearance of the landscape and sometimes creating local opposition. HAWTs require an additional yaw control mechanism to turn the blades toward the wind. In addition, maintenance of the tower, wings, generator and other gear high up above the ground is very expensive. They also create a constant humming noise when rotating making them a problem close to urban or suburban areas.
Vertical-axis wind turbines (or VAWTs) have the main rotor shaft and blades arranged vertically. Key advantages of this arrangement are that the turbine does not need to be pointed into the wind to be effective, they work with wind from any direction. This is an advantage on sites where the wind direction is highly variable. VAWT are often installed nearer to the base on which they rest, such as the ground or a building rooftop; they can also be mounted at top of tall towers with all the gear (winding coil, gear box, and generator) at ground level. This can provide the advantage of easy accessibility to mechanical components for installation and maintenance. Also VAWT designs do not require a yaw mechanism or extra motors to turn into the wind. In addition, VAWTs generally have lower wind startup speeds than HAWTs. Typically, VAWTs start creating electricity at 6 mph winds. VAWTs may be built at locations where taller structures are prohibited. Moreover, VAWTs situated close to the ground can take advantage of locations where mesas, hilltops, ridgelines, and passes funnel the wind and increase wind velocity.
Examples of VAWTs are disclosed in U.S. Pat. Nos. 5,183,386; 5,405,246; 6,242,818; 7,241,105; and 7,303,369, the disclosures of which are all incorporated herein in their entirety. Yet, many if not all of these VAWTs suffer from one or more of the following disadvantages. Most VAWTs produce energy at only 50% of the efficiency of HAWTs in large part because of the additional drag that they have as the back of the blades rotate into the wind. Having rotors located close to the ground where wind speeds are lower due to wind shear, VAWTs may not produce as much energy at a given site as a HAWT with the same footprint or height.